tree 069243b79e90cffeaf20e64c4ddccdbfaa448248
parent 90f78474fbf9c99c52bf6c960bd946e6341ac641
author Willy Tarreau <w@1wt.eu> 1691562768 +0200
committer Willy Tarreau <w@1wt.eu> 1692030239 +0200
encoding latin1

BUG/MAJOR: http: reject any empty content-length header value

The content-length header parser has its dedicated function, in order
to take extreme care about invalid, unparsable, or conflicting values.
But there's a corner case in it, by which it stops comparing values
when reaching the end of the header. This has for a side effect that
an empty value or a value that ends with a comma does not deserve
further analysis, and it acts as if the header was absent.

While this is not necessarily a problem for the value ending with a
comma as it will be cause a header folding and will disappear, it is a
problem for the first isolated empty header because this one will not
be recontructed when next ones are seen, and will be passed as-is to the
backend server. A vulnerable HTTP/1 server hosted behind haproxy that
would just use this first value as "0" and ignore the valid one would
then not be protected by haproxy and could be attacked this way, taking
the payload for an extra request.

In field the risk depends on the server. Most commonly used servers
already have safe content-length parsers, but users relying on haproxy
to protect a known-vulnerable server might be at risk (and the risk of
a bug even in a reputable server should never be dismissed).

A configuration-based work-around consists in adding the following rule
in the frontend, to explicitly reject requests featuring an empty
content-length header that would have not be folded into an existing
one:

    http-request deny if { hdr_len(content-length) 0 }

The real fix consists in adjusting the parser so that it always expects a
value at the beginning of the header or after a comma. It will now reject
requests and responses having empty values anywhere in the C-L header.

This needs to be backported to all supported versions. Note that the
modification was made to functions h1_parse_cont_len_header() and
http_parse_cont_len_header(). Prior to 2.8 the latter was in
h2_parse_cont_len_header(). One day the two should be refused but the
former is also used by Lua.

The HTTP messaging reg-tests were completed to test these cases.

Thanks to Ben Kallus of Dartmouth College and Narf Industries for
reporting this! (this is in GH #2237).

(cherry picked from commit 6492f1f29d738457ea9f382aca54537f35f9d856)
Signed-off-by: Amaury Denoyelle <adenoyelle@haproxy.com>
(cherry picked from commit a32f99f6f991d123ea3e307bf8aa63220836d365)
Signed-off-by: Amaury Denoyelle <adenoyelle@haproxy.com>
(cherry picked from commit 65921ee12d88e9fb1fa9f6cd8198fd64b3a3f37f)
Signed-off-by: Amaury Denoyelle <adenoyelle@haproxy.com>
(cherry picked from commit d17c50010d591d1c070e1cb0567a06032d8869e9)
[wt: applied to h2_parse_cont_len_header() in src/h2.c instead]
Signed-off-by: Willy Tarreau <w@1wt.eu>